A detailed understanding of renal tubular salt transport is a fundamental requirement for understanding regulation of extra cellular fluid volume. This proposal is directed at obtaining a better understanding of these transport processes. We plan to examine the segmental and cellular distribution of membrane molecules responsible for salt transport principally in rabbits and in nephrectomized tissue from humans. Our approach to the problem is to measure the binding of radiolabelled diuretics and ouabain to in vitro tubules. The kinetics of binding will be measured using standard radiochemical assay methods. The membrane location and amount bound will be measured using high-resolution quantitative autoradiography of water soluble compounds. Our initial emphasis will be on ethacrynic acid and furosemide binding in rabbit renal medulla and ouabain binding in human medulla. These measurements will provide a clearer insight into the basic mechanisms of epithelial transport as well as the sites and mechanisms of diuretic action.